1. Field of the Invention
The present invention pertains to a liquid developer for electrophotography that is used to develop an electrostatic latent image in an image forming apparatus such as a copying machine or printer using the electrophotographic method, as well as to an image forming apparatus.
2. Description of the Related Art
Image formation using the electrophotographic method is divided into a dry developing method and a wet developing method.
In dry development, development is performed using toner only or using toner mixed with a carrier. Dry toner comprises mainly a coloring agent, such as a pigment, and a binder resin, to which a charge controller, plasticizer, surface lubricant, etc., are added by means of internal dispersion, or adherence or fixation to the toner particle surfaces. In dry development, toner is charged by means of frictional charging, and is supplied for the purpose of development by means of the electrostatic force of the latent image.
Particles of dry toner that are used in dry development start to float in the air if they are made smaller than a certain size, so they cannot be made too fine. Therefore, relatively large particles, up to approximately 10 micron in average particle diameter, are used. The dry method thus has the technological limitation that the development resolution is not very high due to the relatively large size of the toner particles, as described above.
On the other hand, in wet development, a developer comprising a binder resin on which toner including a coloring agent is dispersed, or a developer comprising a resin that is colored itself, have been proposed. Where necessary, developers on which a charge controller, dispersion promoting agent, etc., are also dispersed have been proposed as well.
Charging of these types of toners is thought to occur based on ion adsorption by the charge controller, and the charged toner is supplied for development based on the principle of electrophoresis.
The toner used in the wet method has the technological advantage over dry method toner that toner particles may be made very small because they will not escape into the air. Therefore, when supplied to an image forming apparatus, said toner offers the advantages that a high-resolution image can be obtained and that fusing of the toner image is easy.
On the other hand, a liquid developer entails the problems of sedimentation of the toner particles in the carrier solution and of imperfect dispersion, which do not exist with dry developers. The inventors of the present invention have focused their attention on this point.
This sedimentation problem is quite significant with regard to liquid developers that have a resin ingredient and include particles of toner of a certain size or larger, i.e., so-called binder toner.